In general, the term OLED is understood to mean an electronic device which contains at least one organic material and which emits light on application of electrical voltage. The basic structure of OLEDs is known to those skilled in the art and described, inter alia, in U.S. Pat. Nos. 4,539,507, 5,151,629, EP 0676461 and WO 98/27136.
The energies of the S1 and T1 states of a compound, in the context of the present application, are defined as those energies which are obtained by quantum-chemical calculations for the states of the compound in question. The S1 state is the energetically lowest-lying excited singlet state, and the T1 state is the energetically lowest-lying triplet state. The exact way in which the quantum-chemical calculations are conducted is described in the working examples.
An amine compound in the context of the present application is understood to mean a compound containing at least one amino group, especially a compound containing at least one arylamino group.
The prior art discloses that it is possible to obtain OLEDs having very good efficiencies with particular purely organic emitting compounds which do not phosphoresce but fluoresce. For example, H. Uoyama et al., Nature 2012, 492, 234, discloses that, with carbazolyl-cyanobenzene compounds as emitting compounds, it is possible to obtain OLEDs having external quantum efficiencies that are similarly good or better than those obtainable with phosphorescent emitters. Such emitting compounds are characterized in that they have a small difference between the energies of the S1 and T1 states. The mechanism of emission on which they are based is referred to as thermally activated delayed fluorescence (TADF). Mehes et al., Angew. Chem. Int. Ed. 2012, 51, 11311, likewise describes the use of compounds that exhibit TADF in OLEDs.
Said documents relate exclusively to OLEDs in which the organic functional layers, especially those layers disposed between the anode and emitting layer, have been applied by gas phase deposition. However, there is great interest in applying TADF emitter technology also to OLEDs having layers applied from solution. There is a particular interest in at least the layers between the anode and emitting layer being applied from solution.
However, the results shown in the prior art are not completely satisfactory. It is especially desirable for the OLEDs based on the TADF emission mechanism to have a high lifetime and very good performance data, especially low operating voltage and high quantum efficiency, and there is a need for improvement in this regard. Moreover, it is of great significance that OLEDs based on the TADF emission mechanism have only a minor proportion of rejects, meaning that only a small proportion of the OLEDs produced, preferably a negligibly small proportion, does not work. This is of particular significance when thin layers of thickness less than 30 nm are applied.